The good argument is that no non-Muslim has any business dictating what is or is not "true Islam". It may be fine to report on one's understanding of what the majority of Muslims seem to be saying on the matter, but to put that in the form of an authoritative declaration is both patronizing and ridiculous.

But the idea that this forces one to accept the self-declared "name" of an organization is also ridiculous. If the LRA had decided to call itself "Christians of Africa" would anyone seriously think it appropriate to go around saying "the Christians of Africa must be destroyed"? Of course it is appropriate to refuse ISIS the dignity of calling it "The Islamic State", and indeed to accept that designation is an insult to the many Muslims who do not accept it as such. (But of course this refusal can and should be done by reference to the requests of other Muslims rather than to one's own position on what is "true Islam".)